Flash phase engineering of MoS2 nanofilms for enhanced photoelectrochemical performance

A heterophase structure combining semiconducting 2H- and metallic 1T-MoS2 exhibits significantly enhanced photoelectrochemical performance due to the electrical coupling and synergistic effect between the phases. Therefore, site-selective effective phase engineering is crucial for the fabrication of MoS2-based photoelectrochemical devices. Here, we employed a flash phase engineering (FPE) strategy to precisely fabricate a 2H-1T heterophase structure. This technique allows simple, efficient, and precise control over the micropatterning of MoS2 nanofilms while enabling site-selective phase transition from the 1T to the 2H phase. The detection of reduced glutathione (GSH) showed an approximately 5-fold increase in sensitivity when using the electrode fabricated by FPE.


Introduction
2][3][4][5] Among them, molybdenum disulde (MoS 2 ) emerges as the most prominent, featuring a metallic 1T phase and semiconducting 2H phase. 6,7While 1T-MoS 2 is a good electrical conductor, it lacks sufficient optical response capabilities.][10][11] These inherent limitations in each phase pose a challenge to their practicality in photoelectrochemical (PEC) devices. 6,10To fully exploit the potential of MoS 2 in PEC applications, it is necessary to couple semiconducting and metallic regions within a single MoS 2 lm by fabricating a 2H-1T heterophase structure.
A direct and effective idea is to drive the partial phase transition in a homophase 1T-or 2H-MoS 2 lm.The in situ transition could be realized by electron injection into a thermodynamically stable 2H-MoS 2 lm or annealing treatment of an active 1T-MoS 2 lm. 12-15However, both phase transition pathways are challenging in terms of precise control of the extent and location of the phase transition, as they always occur simultaneously throughout the entire lm.Therefore, a method to selectively phase transition MoS 2 by a controllable extent and location would enable tunable electrode properties.
In this work, we propose a practical ash phase engineering (FPE) strategy to fabricate 2H-1T heterophase nanolms, achieving a controllable 1T to 2H phase transition.1T-MoS 2 is a good electrical conductor, but it lacks sufficient optical response capabilities.In contrast, 2H-MoS 2 exhibits an exceptional optical response for PEC with photogenerated holes.Glutathione (GSH) can act as an electron donor through oxidization in a PEC process, reacting with the holes located on the 2H-MoS 2 .These electrons can be quickly injected into the conduction band by 1T-MoS 2 .The resulting increase in photocurrent enables quantitative detection of GSH.

Results
A 1T-MoS 2 nanolm was used as the basal substrate, which was obtained via wet-chemistry exfoliation and vacuum ltration with a 25 nm pore size membrane. 14Different nanolm thicknesses were controlled by adjusting the volume of 1T-MoS 2 during the fabrication process.Then, the desired micro-/ nanopattern coupling of two phases was achieved by FPE (Fig. 1).A 488 nm laser beam with a focus diameter of 15 mm (1/ e 2 ) was employed, and its power and scan speed were exibly tuned for precise control. 15,16For example, a logo pattern was achieved by a predened laser scan path (Fig. 2a).A distinct contrast could be observed in the scanning electron microscope (SEM) image before and aer the laser treatment (Fig. 2b).The atomic force microscopy (AFM) image and thickness prole show the ultrathin (nanometer) thickness of the fabricated heterophase MoS 2 nanolm (Fig. 2c).
Raman and UV-vis spectroscopy were used to verify the laserinduced phase transition in MoS 2 .The Raman spectra show characteristic peaks originating from the in-plane (E 1 2g ) and outof-plane (A 1g ) Mo-S phonon modes both before and aer laser treatment (Fig. 2d).However, in the ash phase engineered MoS 2 (FPE-MoS 2 ), the specic peaks corresponding to 1T-MoS 2 are absent, designated as J 1 (153.2cm −1 ), J 2 (225.1 cm −1 ), and J 3 (327.9cm −1 ) modes, indicating a phase transition from 1T-to 2H-MoS 2 . 14,17,18The UV-vis absorption spectrum of FPE-MoS 2 clearly shows 2H-MoS 2 excitonic features (Fig. 2e).A and B excitonic peaks were observed at approximately 616 nm and 671 nm, respectively, while C and D excitonic peaks appeared at around 429 nm. 14o further validate the phase transition, X-ray photoelectron spectroscopy (XPS) was used (Fig. 2f).Two characteristic peaks were observed in both MoS 2 and FPE-MoS 2 at approximately 232.5 eV and 228.9 eV, corresponding to Mo 3d 3/2 and Mo 3d 5/2 , respectively.Additional peaks at 231.4 eV and 228.2 eV could be assigned to 1T-MoS 2 , while peaks at 232.5 eV and 228.9 eV suggest the presence of the 2H phase. 6,19In addition, all Mo 3d and S 2p peaks of the FPE-MoS 2 sample shied to a slightly higher binding energy (Fig. S1 †).The appearance of all of the above features indicates the successful laser-induced 1T to 2H phase transition.
In the FPE process, the incident laser beam irradiates the 1T-MoS 2 nanolm, which serves as an absorbing layer, resulting in a rapid conversion of laser energy into localized thermal energy.Consistent with our previous investigations, the estimated temperature at the center of the laser irradiation spot on each absorber exceeds 343 °C (Fig. S2 †), 20 which triggers the phase transition process. 21The precise location of the laser focus was optimized to hit the nanolm interface (Fig. 3a).
It should be noted that the thickness of the 1T-MoS 2 substrate nanolm also inuences the effect of phase transition, as evidenced by the patterns observed in a simple microline design (Fig. 3b and S3 †).The sharpness/clarity of the lines increases with thicker layers and higher laser power densities, indicating that more MoS 2 undergoes phase transition, until  Paper RSC Advances some defects start to emerge at $0.631 mW mm −2 .This phenomenon is more pronounced in the PEC performance, which is strongly inuenced by the 2H-1T heterophase structure.FPE shows minimal PEC enhancement on the 20 nm 1T-MoS 2 nanolm, while it becomes more pronounced on the thicker 1T-MoS 2 nanolms.In this context, the 1T-MoS 2 nano-lm serves as an effective charge carrier acceptor and transporter, suppressing the recombination of photogenerated charge carriers and facilitating their rapid transport.Consequently, this leads to enhanced PEC performance due to the larger number of charge carriers effectively contributing to the photoelectrochemical process.
In addition, due to the more obvious difference in PEC response between different laser densities, the 35 nm thin 1T-MoS 2 nanolm was used as a model platform to study the effect of the laser-induced ash phase transition and the underlying mechanisms driving the enhancement of PEC performance.As the laser power density increases, a greater amount of 1T-MoS 2 undergoes phase transition to the 2H-MoS 2 (Fig. 4a), as evidenced by the Raman spectra showing weaker peaks in the 1T phase and stronger peaks in the 2H phase (Fig. 4b).The photocurrent response shows an initial increase and then decreases with increasing laser power density, reaching a maximum at 0.277 mW mm −2 (Fig. 4c).This behavior can be mainly attributed to the degree of phase transition to 2H-MoS 2 , which has a high optical absorption but a limited charge transfer capability.Less 2H-MoS 2 (potentially a mono-or few layer transition) was achieved under very low irradiation (0.083 mW mm −2 ), which suffers from low light absorption since it is too thin (as shown in Fig. 4a).In contrast, too much 2H-MoS 2 in the lm hinders electron transport, leading to diminished PEC performance.Thus, the introduction of moderate amounts of 2H-MoS 2 within the 2H-1T heterophase structure provides a favorable balance between optical absorption and efficient electronic transport.In addition, other laser parameters were also studied, such as laser scan speed and line spacing (Fig. S4 †).Specically, the optimized PEC performance was achieved under the laser power density of 0.277 mW mm −2 , 10 mm s −1 laser scan speed, and 0.05 mm line spacing.Additionally, the FPE process results in thermal and morphological folding (Fig. 4d), increasing the specic surface area of the electrode for enhanced optical absorption.
Then, this optimized FPE-MoS 2 structure was used as a photoanode in a PEC device, as shown in Fig. 5a.The results show that the photocurrent of the optimized FPE-MoS 2 electrode exhibits a substantially higher response (Di 1 ) compared to the non-engineered counterpart (Di 2 ) during the on-off cycles of white light irradiation (Fig. 5b).Finally, the optimized PEC device was used for biosensing applications.In this study, reduced GSH was used as a model analyte due to its relevance in various diseases, including aging and cancer. 22,23To evaluate the biosensing activity of the 2H-1T heterophase structure, chronoamperometry (i-t) measurements (Fig. 5c) were conducted.The results show a substantial increase in the photocurrent response with increasing GSH concentration, specically at 100 nM intervals.Upon the addition of GSH to the system, the reducing agent serves as the electron donor, which quickly scavenges the holes, suppressing the recombination of photogenerated charge carriers.These electrons were injected into the conduction band of the MoS 2 and subsequently transferred to the FTO electrode, leading to a relatively large photocurrent response.During this process, GSH is oxidized to glutathione disulde.The 1T-MoS 2 electrode with light was compared with the FPE-MoS 2 electrode with and without light,  which highlights the signicant enhancement in biosensing of the 2H-1T hetero-phase electrode under light irradiation.Further examination revealed that the photocurrent exhibited a linear relationship with physiological GSH concentrations (Fig. 5d).This linear relationship demonstrates high sensitivity (S = Di/Dc), which is approximately ve times higher than that observed in MoS 2 biosensing systems without phase engineering.In addition, the stability was studied with an electrode which was kept at 4 °C, periodically measuring its performance (Fig. S5a †).Aer 7 days, the photocurrent response of the FPE-MoS 2 electrode decreased by less than 13%, indicating good storage stability.Moreover, the reproducibility of the optimized PEC device was veried by measuring the photocurrent response of three electrodes in parallel under successive addition of GSH, showing an RSD of 5.36% (Fig. S5b †).This, indicates a high reproducibility of the FPE-MoS 2 electrode.

Conclusions
This study demonstrates a facile and efficient method to achieve a controllable 1T to 2H phase transition in a MoS 2 nanolm through FPE.The fabrication of a 2H-1T MoS 2 heterophase structure has led to a signicant improvement in PEC performance, which is contributed to the synergistic coupling of metallic 1T-MoS 2 and semiconducting 2H-MoS 2 phases.The inclusion of 2H-MoS 2 as a light absorber and the utilization of 1T-MoS 2 as an excellent electron acceptor and transporter collectively enhance the photoelectrochemical activity of the material.This work introduces a practical strategy to achieve the phase transition and offers promising potential for largearea fabrication of site-selective heterophase structures.Moreover, the ndings open up opportunities for a wide application range, including biosensing.The ability to control the phase transition in MoS 2 through this approach may pave the way for advanced biosensing platforms with improved sensitivity and performance, facilitating early disease diagnostics and other (bio)analytical applications.

Fig. 1
Fig. 1 Schematic presentation of 1T to 2H phase transition via FPE.A 1T-MoS 2 nanofilm serves as an absorbing layer, which rapidly converts laser energy into localized thermal energy, triggering its phase transition.The site selective phase transition was achieved by a predefined laser scanning path.

Fig. 3
Fig. 3 Optimization of laser parameters.Microline pattern fabricated using different (a) focus distances (4.08, 4.18, 4.28 mm from left to right).Here, 4.18 mm was defined as 0 and the other two values are offset by 100 mm (b) different laser power densities of 0.063, 0.227, 0.456, and 0.631 mW mm −2 were screened from left to right.

Fig. 4
Fig. 4 FPE-MoS 2 device with enhanced PEC performance.(a) Optical images of FPE-MoS 2 under different laser power densities for phase transition.The corresponding (b) Raman spectra and (c) photocurrent response with optical images of FPE-MoS 2 electrodes inserted under 0.5 V vs. Ag/AgCl under white light irradiation.(d) Atomic force microscopy image of optimal FPE-MoS 2 , using 0.277 mW mm −2 laser power density, 10 mm s −1 laser scan speed, and 0.05 mm line spacing.

Fig. 5
Fig. 5 Biosensing application of FPE-MoS 2 PEC device.(a) Scheme of the PEC device with MoS 2 on FTO as the photoanode and Pt as the photocathode.(b) Photocurrent response of FPE-MoS 2 (Di 1 ) and MoS 2 (Di 2 ) electrodes under 0.5 V vs. Ag/AgCl under white light irradiation.(c) Amperometric responses of relative electrodes under the same PEC conditions for successive additions of GSH.(d) Corresponding calibration curves for the MoS 2 and FPE-MoS 2 electrodes with white light illumination.